Oxygen generating compositions

ABSTRACT

The oxygen generation composition utilizes magnesium oxide to modify and smooth the oxygen generation rate, suppress chlorine formation, improve the rheology and uniformity of performance of an operating chemical oxygen candle, and to facilitate the mixing of the candle ingredients. The oxygen generation composition typically comprises a metal powder as a fuel; magnesium oxide; and an oxygen source selected from the group consisting of alkali metal chlorates, alkali metal perchlorates, and mixtures thereof. The composition can also include a transition metal oxide catalyst, and can include a binder.

RELATED APPLICATION

This is a continuation of Ser. No. 08/648,293 filed May 15, 1996 nowU.S. Pat. No. 5,882,545.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to oxygen generating compositions, andmore particularly concerns improved oxygen generation compositionscontaining magnesium oxide as an additive to modify and smooth theoxygen generation, suppress chlorine formation, and to facilitate mixingof the compositions.

2. Description of Related Art

Chemical oxygen generating compositions based upon the decomposition ofalkali metal chlorates or perchlorates have long been used as anemergency source of breathable oxygen, such as in passenger aircraft,for example. Oxygen generating compositions utilizing alkali metalchlorates or perchlorates are disclosed in U.S. Pat. Nos. 5,198,147;5,279,761; and 5,298,187; each of which are incorporated herein byreference.

A typical chemical oxygen generating candle may have several layers withdifferent compositions and thus different reaction rates. Multiplelayers are used in the candle instead of a single formula to help matchthe oxygen generation requirements, which vary with time as an airplanedescends following a loss of cabin pressure. Different applications havedifferent oxygen generation requirements. The candle typically has agenerally cylindrical shape with a taper, with a recess at one end tohold an ignition pellet. The ignition pellet is typically ignited byfiring a primer, and heat from the ignition pellet then ignites thereaction of the candle body, generating oxygen.

An aircraft oxygen generator is commonly preprogrammed according to thedescent profile of a given type of airplane, and must meet the minimumoxygen flow requirement at all times during a descent. However, oxygengas formed inside a chemical oxygen generation core or candle mustdevelop sufficient pressure to escape from the core. This effect cancause an uneven and erratic smooth flow of oxygen from an activatedoxygen generation core. In order to meet minimum flow requirementsdespite such variations in oxygen flow from the core, excess weight ofthe chemical oxygen generating composition is commonly used. To minimizeunnecessary weight, particularly in aircraft, it would be desirable toprovide oxygen generating compositions that can facilitate the smoothflow of oxygen from an activated oxygen generation core.

Barium peroxide, lithium peroxide and calcium hydroxide have been usedtogether with cobalt oxide as inhibitors to moderate the performance ofsodium chlorate in oxygen generating compositions. However, bariumperoxide is toxic, so that disposal of expended and scrap oxygengenerators containing barium peroxide can be expensive. Lithium peroxideand calcium hydroxide are very strong inhibitors, so that only smallquantities, such as a fraction of one percent in the oxygen generatingcompositions, for example, can be used, making it relatively difficultto uniformly distribute the inhibitors in the oxygen generatingcompositions. Lithium peroxide also tends to cake when it adsorbsmoisture and carbon dioxide, making mixing even more difficult. For aformulation containing cobalt oxide as a catalyst and lithium peroxideor calcium hydroxide as an additive, a prolonged and thorough mixing iscritical to reduce the variation within each lot, and from one lot tothe next. Decomposition of calcium hydroxide also produces water whichcan be undesirable for some applications. Calcium hydroxide also hasminimal catalytic activity when no other catalyst is present.

Since calcium hydroxide is a relatively strong inhibitor, in oxygengenerating compositions formulated with cobalt oxide and calciumhydroxide, localized regions having a high cobalt oxide concentrationand a low calcium hydroxide concentration occur due to imperfect mixing,and have a far higher decomposition rate than other localized regionswith a low cobalt oxide and a high calcium hydroxide concentration.Lithium hydroxide and calcium hydroxide based formulations of oxygengenerating compositions thus typically exhibit a relatively highvariation of performance within and among lots, due to nonuniformdistribution of the ingredients within the oxygen generatingcompositions. It would be desirable to provide an improved oxygengenerating formulation in which a uniform distribution of ingredients iseasier to achieve, which will produce a greater uniform performance, andwhich is easier to manufacture than conventional formulations. Thepresent invention meets these needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for animproved oxygen generation composition that produces a greateruniformity of performance, a smoother, more uniform rate of oxygengeneration, and is easier to mix and manufacture than comparablechlorate/perchlorate oxygen generation compositions utilizing otherconventional oxygen generating inhibitors. Magnesium oxide (MgO) is usedto modify the reaction, smooth oxygen generation, suppress chlorineformation, improve the rheology of an operating chemical oxygen candle,and facilitate the mixing of the candle ingredients.

The invention accordingly provides for an oxygen generating compositionfor producing a breathable gas upon ignition of the composition,comprising a metal powder as a fuel; magnesium oxide as a chlorineremover and reaction rate modifier; and an oxygen source selected fromthe group consisting of alkali metal chlorates, alkali metalperchlorates, and mixtures thereof. The oxygen generating compositioncan also further comprise a transition metal oxide catalyst, and canoptionally further include a binder. In one preferred embodiment, theoxygen generating composition can comprise an oxygen generatingcomposition for producing a breathable gas upon ignition of thecomposition, comprising: about 0.5-15% by weight of a metal powder as afuel; from zero to about 15% by weight of a transition metal oxidecatalyst; about 0.1-5% by weight magnesium oxide; from zero to about 5%of a binder; and the remainder of an oxygen source selected from thegroup consisting of alkali metal chlorates, alkali metal perchlorates,and mixtures thereof. In another preferred embodiment the oxygengenerating composition can comprise about 1-10% by weight of iron powderas a fuel; about 0.1-15% by weight of a transition metal oxide catalyst;about 0.5-4% by weight of magnesium oxide as a chlorine remover andreaction rate modifier; about 1-5% of a binder; and the remainder of anoxygen source selected from the group consisting of alkali metalchlorates, alkali metal perchlorates, and mixtures thereof.

Magnesium oxide is a relatively weak inhibitor which inhibits thedecomposition of sodium chlorate catalyzed by cobalt oxide, and thus canmodify the reaction and smooth oxygen generation. However, in theabsence of, or in a low concentration of cobalt oxide, magnesium oxidecatalyzes the decomposition of sodium chlorate. Since magnesium oxide isa weaker inhibitor when used with cobalt oxide, and is a catalyst in theabsence of or in a low concentration of cobalt oxide, any variation ofperformance due to non-uniform distribution of cobalt oxide andmagnesium oxide is much reduced. Magnesium oxide is also an anticakingagent, has very good flow properties, and can facilitate mixing.Therefore, the step of mixing of formulations using magnesium oxide asan additive can be greatly simplified in comparison with the prolongedmixing required for conventional oxygen generating compositions.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description, and the accompanyingdrawing, which illustrates by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an oxygen generating candle formedfrom the oxygen generating composition according to the principles ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Flow of oxygen from an activated conventional oxygen generation core orcandle can often be uneven and erratic. Formulations of oxygengenerating compositions using lithium hydroxide and calcium hydroxidereaction rate inhibitors also typically exhibit a relatively highvariation of performance within each lot and among different lots of thesame formulation, due to nonuniform distribution of ingredients withinthe oxygen generating compositions.

The invention is accordingly embodied in an oxygen generatingcomposition for producing a breathable gas upon ignition of thecomposition, which generally comprises a chlorate, a metal powder, abinder, and magnesium oxide. The new magnesium oxide additive is used tomodify the reaction, smooth oxygen generation, suppress chlorineformation, improve the rheology of an operating chemical oxygen candle,and facilitate the mixing of the candle ingredients. The formulation mayoptionally include cobalt oxide and/or other catalysts.

The oxygen generating composition of the invention generally comprisesabout 0.5-15% by weight of a metal powder as a fuel. Preferred metalpowders include iron, tin, manganese, cobalt, nickel, tungsten,titanium, magnesium, aluminum, niobium, zirconium, and mixtures thereof.In one presently preferred embodiment, iron powder having a high purityand a high surface area is used. Suitable iron powder may be obtainedelectrolytically or by hydrogen reduction to yield iron powdersubstantially free of carbon and other impurities which can producetoxic compounds upon operation of the oxygen generating candle. Use ofiron powder in the oxygen generating candle furnishes heat and assistsin stabilizing the chlorate decomposition.

In another preferred embodiment, tin powder having high purity and smallparticle size is used. Tin powder with particle size smaller than 325mesh is preferred even though coarser tin powder also works in asatisfactory way. Tin powder is generally free of carbon. Therefore,when tin is used the contamination of carbon monoxide in the oxygengenerated is minimized.

In a presently preferred embodiment, the oxygen generating compositionfurther comprises about 0.1-15% by weight of a transition metal oxidecatalyst. The transition metal oxide catalyst can, for example, beselected from the group consisting of oxides of cobalt such as CoO, Co₃O₄, Co₂ O₃, and CoO_(x), where x equals 1.0 to 1.5, nickel oxide, copperoxide, iron oxide, and mixtures thereof. The presently preferredtransition metal oxide catalysts are cobaltocobaltic oxide (Co₃ O₄),nickel oxide and copper oxide.

There are advantages to using a reaction rate inhibitor when cobaltoxide is used, because cobalt oxide is so active that the decompositionreaction occurs in the solid phase. By reducing the catalytic activity,the reaction temperature can be raised slightly, so that the reactionoccurs in a viscous molten phase. This permits smooth release of theoxygen gas generated through the decomposition of the chlorate orperchlorate.

In a currently preferred embodiment, the oxygen generating compositionfurther includes from about 0.1 to about 5% by weight of magnesium oxideas a reaction rate inhibitor. In oxygen generating compositionsformulated, for example, with cobalt oxide and calcium hydroxide, whichacts as a relatively strong reaction inhibitor, localized regions havinga high cobalt oxide concentration and a low calcium hydroxideconcentration can occur due to imperfect mixing, and would have a farhigher decomposition rate than localized regions with a low cobalt oxideand a high calcium hydroxide concentration. Magnesium oxide, on theother hand, is a relatively weak inhibitor which inhibits thedecomposition of sodium chlorate catalyzed by cobalt oxide, and thus canmodify the reaction and smooth oxygen generation. However, in theabsence or low concentration of cobalt oxide, magnesium oxide catalyzesthe decomposition of sodium chlorate. This is particularly advantageousfor moderating the reaction rate of the oxygen generating composition.Since magnesium oxide is a weaker inhibitor when used with cobalt oxide,and is a catalyst in the absence of cobalt oxide, non-uniform mixing ofthe oxygen generating composition will not so adversely affectuniformity of performance of the oxygen generating composition, incomparison with prior conventional oxygen generating compositions.

Magnesium oxide is also advantageous in that it is very fluffy, so thatit can also function as a thickening agent to help improve the moltenrheology of a working oxygen generator. Sodium chlorate based oxygencandle compositions commonly have operating temperatures above themelting point of sodium chlorate, at about 265° C., so that the sodiumchlorate in the oxygen generating composition can melt and form a paddleduring operation of the oxygen generator. Proper operation of a chemicaloxygen candle depends on the propagation of the reaction front along thelongitudinal axis of the candle to release oxygen uniformly according toa predefined profile. Once a region within the candle melts and forms apaddle, the reaction can go in a much faster and irregular way. Theperformance of an oxygen candle can therefore be improved by improvingits melt rheology. Since magnesium oxide is a moderately active catalystin the absence of cobalt oxide, and is a weak inhibitor in the presenceof cobalt oxide, the variation due to uneven distribution is muchsmaller when magnesium oxide is used in place of calcium hydroxide.Evidence of this benefit is the observation that the expended blocks ofthe oxygen generating composition including magnesium oxide generallyhave more uniform texture. Magnesium oxide also does not produce waterlike calcium hydroxide, and is therefore advantageous for an applicationthat requires low moisture levels in the evolved oxygen.

Magnesium oxide is suitable as an additive in the oxygen generatingcomposition of the invention as long as the material has a smallparticle size and a high surface area. Examples of suitable forms ofmagnesium oxide are the so-called light magnesium oxide available fromAldrich Chemical, and the high surface area magnesium oxide availablefrom C. P. Hall under the commercial name "MAGLITE", although otherforms of magnesium oxide may also be suitable. The light magnesium oxidefrom these two suppliers typically has a surface area of approximately100 to 200 m² /g. Even though magnesium oxide having a high surface areais preferred, magnesium with a lower surface area would also work in asimilar manner, although with a slightly higher loading.

The oxygen generating compositions of the invention can also optionallyfurther include up to about 5% of one or more binders. In a presentlypreferred embodiment, the binder can be an inorganic binder such asglass powder, glass fiber, fiberglass, ceramic fiber, steel wool,bentonite, kaolinite and mixtures thereof, for example, although otherinorganic binders can also be suitable.

The remainder of the oxygen generating composition preferably comprisesan oxygen source selected from the group consisting of alkali metalchlorates, alkali metal perchlorates, and mixtures thereof. The alkalimetal chlorate can be sodium chlorate, potassium chlorate, or lithiumchlorate, for example, and the alkali metal perchlorate can be potassiumperchlorate, lithium perchlorate, or sodium perchlorate, for example,although other alkali metal chlorates and perchlorates may be suitable.The presently preferred chlorate is sodium chlorate, and the presentlypreferred perchlorate is potassium perchlorate. Sodium chlorate ispresently preferred as an oxygen source because it has a relatively highoxygen yield per unit weight compared to potassium chlorate, and areasonably low decomposition temperature compared to potassium chlorateand perchlorate and lithium perchlorate. Decomposition of sodiumchlorate is exothermic once initiated, which permits a self-sustainingoperation of an oxygen generating candle or core formed from the oxygengenerating compositions of the invention. Sodium chlorate is alsocurrently preferred as the oxygen source due to its currently relativelylow price. In one presently preferred aspect of the invention, theoxygen source can alternatively be a combination of a major amount ofsodium chlorate and a minor amount of potassium perchlorate.

In one presently preferred embodiment of the invention, the oxygengenerating composition can comprise about 1-10% by weight of metalpowder as a fuel; about 0.1-15% by weight of a transition metal oxidecatalyst; about 0.5-4% by weight of magnesium oxide as a chlorineremover and reaction rate modifier; about 1-5% of a binder; and theremainder of an oxygen source selected from the group consisting ofalkali metal chlorates, alkali metal perchlorates, and mixtures thereof.

In forming an oxygen generating core or candle, the metal powder, metaloxide catalyst, magnesium oxide, and the inorganic binder (if used) arepremixed. The oxygen source chlorate/perchlorate component is typicallyseparately mixed with approximately 1 to 5% water, by weight, which isused as a lubricant to facilitate the formation of the oxygen generatingcores or candles. The premixed powder is then mixed with the wetchlorate/perchlorate. The chemical oxygen candles are formed bycompaction of the damp mixture in a mold, and are then dried at about120° C. to remove the water that was added during the mixing process.However, a small residue of water may remain in the oxygen generatingcomposition even after drying.

With reference to FIG. 1, a typical chemical oxygen generating candle 10commonly is composed of several layers, with each layer having adifferent formulation. The multiple layers with different formulationscan thus be designed to match the desired oxygen generation rate, basedupon specified requirements of the application, since differentapplications may have different oxygen generation rate requirements.While the oxygen generating candle shown in FIG. 1 has 5 layers, anynumber of layers can be used to form the oxygen generating candle. Ironpowder and tin powder are currently preferably used as a fuel in one ormore of the layers, depending upon the application. The various types ofinterface shapes between layers, shown in FIG. 1, are used to helpcontrol the transition of the reaction as it progresses from one layerto another. The interface shapes and relative sizes and reactivities ofthe layers can be modified, depending upon the requirements of thespecific applications of the oxygen generating candles. Oxygengeneration candles or cores are typically formed in a cylindrical shapewith a taper. At the top of the candle there is a recess to hold anignition pellet 12, which can be ignited by firing a percussion primer,for example. A typical ignition pellet can, for example, have acomposition of about 35% by weight iron, 13-15% by weight cobalt oxide,about 5% iron oxide, about 3-4% by weight of a binder, and the balancesodium chlorate. The heat from the ignition pellet is then sufficient toinitiate the decomposition of the layers 14, 16, 18, 20 and 22 of theoxygen generating candle to release oxygen.

The oxygen generating compositions of the invention are furtherillustrated in the following examples, in which percentages are byweight.

EXAMPLE 1

    ______________________________________                                        1st layer: 17 grams: 12.5% iron powder, 9% cobalt                                oxide, 4% glass powder, and 74.5% NaClO.sub.3.                               2nd layer: 58 grams: 6.7% iron powder, 1.45% cobalt                            oxide, 1% magnesium oxide, 2.5% glass                                         powder, and 88.35% NaClO.sub.3.                                              3rd layer: 80 grams: 5.5% iron powder, 0.55% cobalt                            oxide, 1% magnesium oxide, 2% glass powder,                                   and 90.95% NaClO.sub.3.                                                      4th layer: 70 grams: 3.4% iron powder, 0.25% cobalt                            oxide, 1% magnesium oxide, 2% glass powder,                                   and 93.35% NaClO.sub.3.                                                      5th layer: 145 grams: 2.3% iron powder, 0.18% cobalt                           oxide, 1% magnesium oxide, 2.5% glass                                         powder, and 94.02% NaClO.sub.3.                                            ______________________________________                                    

The powders for each layer were premixed without sodium chlorate, thesodium chlorate was mixed with a small amount of distilled water, andthen the premixed powders were mixed with the wet sodium chlorate.Chemical oxygen candles were then formed by compaction of the dampmixtures in a mold. The candles were then dried at 120° C. to remove thewater added.

A dried candle was then loaded into a stainless steel housing, ignited,and operated for 14.5 minutes, generating approximately 111.3 liters ofoxygen at room temperature. The reaction was uniform and the expendedcandle had a uniform texture. No magnesium oxide was used in the firstlayer, to insure a very high initial oxygen generation rate. Loading ofmagnesium oxide in the first layer would slightly reduce the initialoxygen generation rate, which would be suitable for some applications.

EXAMPLE 2

    ______________________________________                                        1st layer:                                                                              21.2 grams: 10.0% tin powder, 8.5% cobalt                              oxide, 4.0% glass powder, and 77.5% sodium                                    chlorate.                                                                    2nd layer: 32.3 grams: 6.5% tin powder, 0.8% MgO, 1.0%                         cobalt oxide, 2.0% glass powder, and 89.7%                                    sodium chlorate.                                                             3rd layer: 83.5 grams: 5.5% tin powder, 0.5% MgO,                              0.48% cobalt oxide, 2.0% glass powder, and                                    91.54% sodium chlorate.                                                      4th layer: 98.3 grams: 5.0% tin powder, 0.5% MgO,                              0.37% cobalt oxide, 2.0% glass powder, and                                    92.13% sodium chlorate.                                                      5th layer: 378.8 grams: 4.0% tin powder, 0.5% MgO,                             0.25% cobalt oxide, 2.0% glass powder, and                                    93.25% sodium chlorate.                                                    ______________________________________                                    

The ingredients were mixed in the same way as stated in example 1. Acandle was pressed, dried, loaded and tested in the same way asdescribed in example 1. The candle operated 16.22 minutes and generatedapproximately 191.1 liters of oxygen.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. For instance, although water was utilized in forming thecompositions in the examples above, it would be possible to suitablycompress dry mixtures to form chemical cores. Furthermore, although thecores as illustrated are typically cylindrical with a taper, molds couldbe constructed in many other configurations. Accordingly, it is notintended that the invention be limited, except as by the appendedclaims.

What is claimed is:
 1. An oxygen generating composition for producing abreathable oxygen gas for emergency use for sustaining life for severalminutes upon ignition of the composition, the composition consistingessentially of:a metal powder fuel; magnesium oxide; a binder; and theremainder consisting essentially of an oxygen source selected from thegroup consisting of alkali metal chlorates, alkali metal perchlorates,and mixtures thereof, said composition being capable of producing saidbreathable oxygen gas for a period of several minutes for sustaininglife.
 2. The oxygen generating composition of claim 1, wherein saidoxygen source is an alkali metal chlorate selected from the groupconsisting of sodium chlorate, potassium chlorate, lithium chlorate, andmixtures thereof.
 3. The oxygen generating composition of claim 1,wherein said oxygen source is an alkali metal perchlorate selected fromthe group consisting of potassium perchlorate, lithium perchlorate,sodium perchlorate, and mixtures thereof.
 4. The oxygen generatingcomposition of claim 1, wherein said binder is an inorganic binderselected from the group consisting of glass powder, glass fiber, ceramicfiber, steel wool, bentonite, kaolinite and mixtures thereof.
 5. Theoxygen generating composition of claim 1, wherein said fuel is a metalpowder selected from the group consisting of iron, tin, manganese,cobalt, nickel, tungsten, titanium, magnesium, aluminum, niobium,zirconium, and mixtures thereof.
 6. The oxygen generating composition ofclaim 1, wherein said magnesium oxide has a surface area ofapproximately 100 to 200 m² /g.
 7. An oxygen generating composition forproviding an emergency source of breathable oxygen gas for severalminutes upon ignition of the composition, consisting essentially of:ametal powder fuel; a transition metal oxide catalyst; magnesium oxide; abinder; and the remainder consisting essentially of an oxygen sourceselected from the group consisting of alkali metal chlorates, alkalimetal perchlorates, and mixtures thereof, said composition being capableof producing said breathable oxygen gas for a period of several minutesfor sustaining life.
 8. The oxygen generating composition of claim 7,wherein said transition metal oxide catalyst is selected from the groupconsisting of cobalt oxide, nickel oxide and copper oxide.
 9. The oxygengenerating composition of claim 7, wherein said oxygen source is analkali metal chlorate selected from the group consisting of sodiumchlorate, potassium chlorate, lithium chlorate, and mixtures thereof.10. The oxygen generating composition of claim 7, wherein said oxygensource is an alkali metal perchlorate selected from the group consistingof potassium perchlorate, lithium perchlorate, sodium perchlorate, andmixtures thereof.
 11. The oxygen generating composition of claim 7,wherein said magnesium oxide has a surface area of approximately 100 to200 m² /g.
 12. The oxygen generating composition of claim 7, whereinsaid fuel is a metal powder selected from the group consisting of iron,tin, manganese, cobalt, nickel, tungsten, titanium, magnesium, aluminum,niobium, zirconium, and mixtures thereof.
 13. The oxygen generatingcomposition of claim 7, wherein said binder is an inorganic binderselected from the group consisting of glass powder, glass fiber, ceramicfiber, steel wool, bentonite, kaolinite and mixtures thereof.